1. Field of the Invention
The present invention relates to a method of preventing drill string overflow, and more particularly, but not by way of limitation, to an improved method of lowering the level of drilling fluid in a drill string below the point at which drill pipe sections are removed therefrom.
2. Description of the Prior Art
In the rotary drilling of well bores penetrating subterranean formations utilizing drill strings suspended in the well bores, it is frequently necessary to withdraw the drill strings from the well bores to replace worn-out drill bits and/or to carry out other remedial or treating procedures.
During the drilling of a well bore, drilling fluid is circulated from the surface through the drill string, through ports or nozzles in the drill bit attached to the lower end of the drill string and back to the surface by way of the annulus between the drill string and the walls of the well bore. When drilling is stopped for the removal of the drill string from the well bore, because the drilling fluid in the annulus contains cuttings from the formation being drilled, it has a higher density than the drilling fluid contained within the drill string. This difference in drilling fluid density causes the level of the drilling fluid in the drill string to be higher than the level of drilling fluid in the annulus. Further, the ports or nozzles in the drill bit at the bottom of the drill string restrict the rate of drainage of drilling fluid from the drill string as it is raised whereby the level is not immediately appreciably lowered therein. The combination of the above factors results in the level of drilling fluid in the drill string usually always being above the rig floor which in turn causes drilling fluid to overflow at the point of drill string disconnection. Such overflow is undesirable for the reasons, among others, that the drilling fluid which overflows is lost, unsafe working conditions for personnel are created, and pollution of the environment results.
When a drill string is removed from a well bore for any reason, it is usually always necessary that the well bore and the portion of the drill string suspended therein remain full of drilling fluid in order to prevent formation fluid blowouts. That is, when the well bore penetrates formations containing formation fluids under pressure, the column of drilling fluid in the well bore extending from the formation to the surface prevents the formation fluids from blowing out. The drilling fluid is maintained at a predetermined density whereby a higher pressure is produced on the formation by the hydrostatic head of the drilling fluid than the pressure of the fluids in the formation. This pressure differential prevents the formation fluids from entering the well bore and blowing the drilling fluid out of the annulus, etc.
In order to prevent blowouts as a result of drilling fluid overflow and the resulting loss of hydrostatic pressure exerted within the well bore by the column of drilling fluid therein, various procedures have heretofore been followed. In one procedure, a quantity of very high density drilling fluid is prepared, e.g., 40 to 50 sacks of weighting material is added to 30 to 50 barrels of drilling fluid and the resulting high density fluid is pumped into the drill string to thereby create an imbalance in density between the inside and outside drilling fluid columns. The presence of the high density fluid within the drill string is intended to cause the level therein to be lower than the column of drilling fluid in the annulus and lower than the drilling rig floor whereby overflow is prevented or reduced. This procedure is less than satisfactory for the reason that it is expensive and time-consuming to carry out and can result in high hydrostatic pressures being exerted in the well bore which cause adverse results. For example, too high a hydrostatic pressure can cause one or more formations penetrated by the well bore to fracture which in turn can cause a sudden drilling fluid loss to the formations and a blowout to occur as a result thereof.
Another procedure which has been followed is to simply put up with drilling fluid overflow at the rig floor when the drill string is being removed and to continuously make up drilling fluid into the well bore to offset the loss of the overflowing drilling fluid and prevent a blowout.
Yet another commonly followed, but more dangerous procedure has been to contend with overflow when removing the drill string without adding additional drilling fluid to the well bore. The level of drilling fluid in the well bore is lowered by the overflow and the overflow subsequently stops whereby the portion of the drill string remaining in the well bore is removed without overflow. This procedure obviously risks a blowout in that the height of the column of drilling fluid remaining in the well bore when the overflow at the drilling rig floor is stopped is unknown.
Methods have heretofore been developed and used for expediting the draining and equalization of drilling fluid which involves the injection of pressurized air or other gas into the drill string to lower the level of drilling fluid therein. While a number of such methods have been effective, they have not received wide acceptance partially because of the large quantities of gas required for displacing the drilling fluid and/or the relatively long periods of time required to achieve such displacement.
By the present invention an improved method of preventing drilling fluid overflow when raising a drill string suspended within a well bore and removing drill pipe sections therefrom is provided which is simple and requires very little time to carry out.